The invention relates to methods of investigating earth formations traversed by boreholes. More particularly, the invention relates to processing well logging data derived from apparatus lowered in a borehole for investigating subsurface earth formations. Still more particularly, the invention relates to verifying and calibrating logs and to reconstructing missing logs.
Boreholes are drilled into the earth in search of earth formations bearing fossil fuels in the form of coal or hydrocarbons, minerals such as sulfur and salt, and water which is pottable or contains valuable salts. A knowledge of formation characteristics is required to locate and evaluate such earth formations. Important characteristics are the lithology or mineral composition of the formations, the grain structure of the formations, the porosity or volume of pore space between the grains, the contents of these pore spaces, the permeability or capacity for fluid flow between the pore spaces, and the structure of the formation which determines its capacity to trap and accumulate significant amounts of deposits. In order to be of a value, a formation must have the correct combination of porosity, permiability, lithology, pore structure and pore contents. A general reference of formation characteristics and their evaluation is a book by E. J. Lynch entitled "Formation Evaluation" and published by Harper and Row in 1962.
An effective method of obtaining information about the characteristics of the earth formations penetrated by boreholes is by investigating apparatus especially adapted for the environment of a borehole. Such apparatus is lowered in the borehole on a wire line after the hole is drilled. This method is known as wireline logging, or more simply well logging. In well logging, the exploring apparatus is electrically and mechanically connected by means of the wire line to control and recording equipment located at the surface. The investigating apparatus is lowered in the borehole by means of a winch and is then withdrawn slowly while deriving measurements versus depth. These measurements are recorded on a strip of film or paper, with the long dimension of the strip scaled in depth, thus forming a log of the borehole measurement versus depth. Alternately, the measurements may be recorded in a suitable electronic memory. The exploring apparatus employed to make the measurements and to generate the electrical signals for recording of these measurements is generally of three types, defined by the method employed to make the measurements. The three types of exploring apparata are electrical, sonic and radiation apparata. A discussion of such well logging techniques may be found in a book by Hubert Guyod and Lemay Shane entitled "Geophysical Well Logging" and published in 1969 for Hubert Guyod, Houston, Tex. A discussion of log interpretation may be found in a book by S. J. Pirson, entitled "Handbook for Well Log Analysis" and published by Prentice-Hall in 1963.
Although general log accuracy is progressively improving with time, modern interpretation techniques create new and very stringent requirements for accuracy. This is particularly true of the porosity logs (neutron, density and sonic), but is also true of other logs (such as spontaneous potential logs, resistivity logs, resistivity micrologs, laterologs, electromagnetic logs, various radioactivity logs and logs such as dip, acoustic, geophone, geothermal, photoelectrical and geochemical logs). It may well be that complex log interpretation techniques, such as shale sands and lithology-porosity interpretation techniques require accuracy which is better than what can be reasonably expected from field recalibrations of measurement devices.
The importance and desirability of log calibration has been recognized in the past, and various techniques have been developed for calibration of logs. For example, manual calibration of logging systems by reference to standards is discussed in an article by Cochrane, J. E., entitled "Principles of Log Calibration and Their Application to Log Accuracy", and published in the Journal of Petroleum Technology in July 1966 at pages 81 through 826. The technique involves one-point, two-point, or multipoint surface calibration by means of reference signals. In particular, the investigating apparatus which is normally lowered into a borehole to take measurements is placed at the surface in an environment whose characteristics are known, and the difference between the expected and the actual signals generated by the apparatus are recorded at one or more points on its response curve. These differences are used to correct the actual measurements taken by the apparatus in a borehole. A similar technique is discussed in an article by Maciula, E. A. and Cochrane, J. E. entitled Quantitative Use of Calibration Data to Correct Miscalibrated Well Logs and published in the Journal of Petroleum Technology in July 1968 at pages 663-670. The technique involves the use of reference signals to determine the offset of a log from an accurate log, and uses this offset as an operator on recorded measurements to convert them to true values.
Another technique is discussed in Jeffries, F. S. and Kemp, E. M., Computer Reconciliation of Sonic Log and Core Analysis in the Boundary Lake Field, Fourth Annual Logging Symposium Transactions, May 23-24, 1963, Oklahoma City, Okla., pages IV-1 through IV-18. The technique involves calibration of sonic legs by reference to porosity data obtained from core analysis.
Another manual approach to calibration of logs is through statistical analysis of logs by a person skilled in log interpretation. This approach involves creating a model of a field, and deciding if a small shift of one or several of the logs from that field would cause them to conform better to the model. The model is the subjective belief of a person who is highly skilled in log interpretation as to what may be the lithology of a particular field. It may be created on the basis of manipulation of porosity readings, as discussed in Burk, J. A. et al., The Litho-Porosity Cross-Plot, SPWLA, Tenth Annual Logging Symposium, May 25-28, 1969, or on the basis of other information such as individual logs from boreholes in the field of interest, or other knowledge of that field. This manual approach requires a person who is highly skilled and highly experienced in interpreting logs (and geological data in general), and involves a high degree of subjectivity. The approach is time-consuming. Additionally, its extension to more than two or three logs is extremely difficult, and emphasizes the importance of the human factor which is necessarily involved in it. This is undesirable, because the use of a greater number of logs adds additional valuable information.